Phase transition characteristics of an organic material, 9,10-di(2-naphthyl)anthracene(ADN), used for Organic Light Emitting Diode(OLED) is evaluated under vacuum. The phase transition shows up as plateau in a temperature curve of ADN material when it is heated up to its melting or sublimation temperature at a pressure in vacuum chamber. At 1 atmospheric pressure the material has a melting temperature that is verified by Differential Scanning Calorimetry(DSC). As pressure is lowered from 1 atmospheric pressure, the melting temperature decreases by a few degree and it starts to sublimate below 10 torr where triple points exist. The sublimation temperature slightly decreases with lowering pressure as well. The result may provide not only the optimal evaporation condition of ADN but also the information of its thermal stability. In a similar way, the phase transition and thermal stability of other various organic materials for flexible electronics and OLED can be estimated under high vacuum.